The cryogenic separation of air is a well established industrial process. Cryogenic air separation involves the filtering of the feed air to remove particulate matter and compression of that feed air to supply the energy required for separation. Following the compression the feed air is cleaned of high boiling impurities such as carbon dioxide and water vapor, cooled, and then separated into products by cryogenic rectification. The separation columns are operated at cryogenic temperatures to allow the gas and liquid contacting necessary for separation by distillation, and the separated products are then returned to ambient temperature conditions against the cooling feed air stream.
The most common cryogenic air separation system for the production of oxygen is the double column system which employs a higher pressure column and a lower pressure column in heat exchange relation at a main condenser. In this system the head pressure is the pressure discharge at the base load air compressor which is set by the pressure at the bottom of the higher pressure column plus the pressure drop in piping and apparatus between the base load air compressor and the higher pressure column. In turn, the pressure at the bottom of the higher pressure column is set by the pressure drop of the stream from the top of the lower pressure column to the atmosphere, by the added pressure difference to the bottom of the lower pressure column, by the temperature difference across the main condenser which sets the high pressure nitrogen condensing pressure at the top of the higher pressure column, and by the added pressure drop to the bottom of the higher pressure column. In conventional systems the pressure at the bottom of the higher pressure column is generally within the range of from 70 to 80 pounds per square inch absolute (psia) resulting in a head pressure generally within the range of from 77 to 87 psia.
The conventional double column system enables the separation of air with good energy efficiency and excellent product purity. However, when lower purity oxygen, i.e. oxygen having a purity of 99 mole percent or less, is desired, the conventional system is less efficient because it has more air separation capability than is being utilized. Since the demand for lower purity oxygen is increasing in applications such as glassmaking, steelmaking and energy production, it is desirable to have a double column system which can produce lower purity oxygen at lower operating costs.
Accordingly, it is an object of this invention to provide an improved double column cryogenic rectification system for producing lower purity oxygen.